Process for the treatment of raw materials containing arsenic and metal

ABSTRACT

A process for the treatment of raw materials which contain arsenic and metal to produce a metal-free arsenic product and an arsenic-free metal sulfate is disclosed, in which the raw material is leached by using an aqueous solution of sulfuric acid and then the metals are crystallized as sulfates from the separated aqueous solution, which is separated from the metal sulfate crystals. The leach is performed at elevated temperature under oxidizing conditions, in order to bring the arsenic to a 5-valent form, the metal sulfates are crystallized out from the selectively separated, arsenate-bearing aqueous solution by cooling, and arsenic is removed from at least part of this aqueous solution before the aqueous solution is returned to the leach.

BACKGROUND OF THE INVENTION

The present invention relates to the treatment of raw materials whichcontain arsenic and metal, in order to produce a metal-free arsenicproduct and an arsenic-free metal sulfate, by leaching the raw materialwith an aqueous solution of sulfuric acid and by crystallizing themetals in the form of sulfates from the separated aqueous solution,which is then separated from the metal sulfate crystals.

The process according to the invention can be used for the treatment ofany arsenic- and metal-bearing raw materials the metals of whichdissolve in a sulfuric acid solution in the form of sulfates and thearsenic of which can be recovered as a metal-free arsenic product, suchas a solution of H₃ AsO₄ or NaAsO₂, or solid As₂ O₃. Suitable rawmaterials include metal arsenides and arsenates, such as the arsenidesor thioarsenides of cobalt and nickel, and arsenic matte. A raw materialespecially suitable for treatment by the process according to theinvention it the arsenic-bearing precipitate produced in theelectrolytic zinc process; it is obtained by cementation by means ofzinc powder and by the addition of some arsenic compound in order tocement the cobalt and nickel out from the zinc sulfate solution. As aresult of this purification process a cementate is obtained in which thecobalt and nickel are in the form of arsenides (MeAs) and which alsocontains a small guantity of copper arsenides and metallic copper. Thispurification process is described in U.S. Pat. No. 3,979,266.

Cementates of the above type, produced by solution purification, used tohave a high copper content and were therefore regarded as raw materialsfor copper production. Owing to their high arsenic content these rawmaterials were not, however, very desirable raw materials for smeltingplants. There was another disadvantage in that the arsenic was wasted,at least as regards the zinc plants. U.S. Pat. No. 3,979,266 discloses aprocess by which it is possible to lower considerably the copperconcentration in the arsenide cementate, and consequently the cementatecan be used as raw material in the production of cobalt and nickel.However, in principle this cementate causes the same problem as above.The arsenic content in the cementate substantially lowers the sellingvalue of the cementate, while the arsenic present in the cementate iswasted as regards a zinc plant. Various processes have been introducedfor the treatment of these cementates. U.S. Pat. No. 3,979,266 disclosesa process of leaching the arsenic in sodium hydroxide and returning itas As₂ O₃ to the solution purification, while the metals are recoveredin the form of a hydroxide precipitate. The precipitate has, however,proved to be difficult to filter and wash. Finnish Lay-Open Print 770646describes another process, in which the zinc, copper, and cobalt arefirst leached in acid and then a cobalt-rich precipitate is precipitatedfrom this solution. The residue is then leached by means of alkali lyeto remove the arsenic. The residue is a suitable raw material for theproduction of copper, and the arsenic is precipitated from the solutionas copper arsenate, which is returned to solution purification.

There is also a prior known method for the treatment of arsenides,whereby arsenides are extracted by means of a sulfuric acid solution andthe solution is thereafter evaporated so as to precipitate the metalsulfates and to recover the arsenic as a sulfuric acid-bearing H₃ AsO₄solution (Journal of the South African Institute of Mining andMetallurgy, July 1969, p. 654). It is, however, difficult to separatearsenic and metal sulfates effectively from each other by this process,since a very strong solution of sulfuric acid and arsenic acid must beused for a complete precipitation of the metal sulfates. This results ina very viscous solution, which complicates the separation and thewashing of the crystal mass.

The object of the present invention is therefore to provide a processfor the treatment of arsenic- and metal-bearing raw materials to producea metal-free arsenic product and an arsenic-free metal sulfate, wherebythe disadvantages involved in the previously known processes mentionedabove are eliminated.

SUMMARY OF THE INVENTION

According to the invention, raw materials which contain arsenic andmetal are leached at an elevated temperature and eventually underoxidizing conditions to convert the arsenic to a 5-valent form,whereafter the metal sulfates are crystallized out by cooling from aselectively separated arsenate-bearing aqueous solution, and arsenic isremoved at least from a secondary flow of the aqueous solution beforethe aqueous flow is returned to the leaching operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the process according to the invention it is not necessary tocrystallize the metal sulfates completely out from the separatedarsenate-bearing aqueous solution, firstly since a closed solution cycleis used, and secondly, since arsenic can be separated very selectivelyfrom the mother liquor after the crystallization. The removal of arsenicfrom this mother liquor need not be complete, either, since after theremoval of arsenic the solution is returned to the leaching of rawmaterial. Thus the present invention comprises the leaching of arsenic-and metal-bearing raw materials by means of an aqueous solution in aclosed solution cycle which includes, in addition to the leach, twoselective removal stages, one for metal sulfates and the other forarsenic.

The invention is described below in more detail with reference to theaccompanying drawing, which depicts a flow diagram of the processaccording to the invention.

Arsenide precipitate is fed to the leaching stage, at which a suitablesulfuric acid concentration is maintained and oxygen is dispersed in theform of either gaseous oxygen or air into the aqueous solution. Thesulfuric acid concentration is not critical for the success of theprocess, for it affects only the reaction velocity, but since a highsulfuric acid concentration enhances both the reaction velocity and theperformance of both of the following removal stages, the sulfuric acidconcentration is maintained at a rather high level, and approx. 100 g H₂SO₄ /l has experimentally been found suitable. The leaching temperatureis selected as one at which the solubility of the metal sulfates ismaximal. Temperatures in the range of about 50°-100° C. are generallyemployed. In leaching a raw material in which the metals are copper andnickel, the temperature is preferably 90°-100° C., while the highestsolubility of cobalt sulfate is at approx. 65°-70° C. When treating anarsenide precipitate from a zinc process, the precipitate containing allthe three metals, the temperature is a compromise between theabove-mentioned temperatures, depending on the predominant metal.

At the end of the leach the solution is cooled or the solution isallowed to cool as much as is practical. It can be cooled to approx. 30°C., for example, which is fully possible by means of generally availablecooling water. Thereby the metal sulfates crystallize. Fresh sulfuricacid is added preferably to this stage of the solution cycle, since ahigh sulfuric acid concentration curbs the coprecipitation of arsenicwith the sulfates, while more sulfates can be precipitated as theirsolubility lowers with increasing acid concentration. The sulfatecrystals are removed by filtering, and then washed.

The next stage is arsenic removal, which can be performed by differentmethods. Two methods are described below, and the selection between themdepends on the local conditions. It has been shown that H₃ AsO₄ can veryselectively be extracted by means of tributyl phosphate or dibutylbutylphosphate from the solution obtained from the crystallization. Theextraction is performed without adding any neutralizing agents orforeign chemicals. A high sulfuric acid concentration in the solutionpromotes, however, the extraction, and therefore the addition of freshsulfuric acid to the crystallization stage is advantageous even at thisstage. The extraction is performed using tributyl phosphate ordibutylbutyl phosphate dissolved in some suitable hydrocarbon to whichsome higher alcohol may have been added to enhance the separation of thephases. The stripping is performed with water. The extraction is easy toperform so that the output H₃ AsO₄ solution contains at minimum 100 gAs/l, and thereafter this solution can be returned to solutionpurification. Thus the arsenic can be kept in the aqueous solutionthroughout the process, which is highly hygienic.

In another method for the removal of arsenic, arsenate is reduced toarsenite, whereby the As₂ O₃ precipitates. The reduction can beperformed using, for example, sulfur dioxide. Thereafter the As₂ O₃ canbe returned to the solution purification, either as such or as anenvironmentally less hazardous NaAsO₂ solution, which is obtained bydissolving the precipitated As₂ O₃ in sodium hydroxide after the motherliquor from the As₂ O₃ precipitation has been removed by decanting.

Above, the process according to the invention has been described asapplied to the treatment of arsenide cementate obtained from zincplants. It is, however, evident, that even other raw materials can betreated by it. The main thing is that arsenic can be dissolved in a5-valent form (=as arsenate) to make possible the selectivecrystallization of sulfates. If the arsenic is in a 3-valent form, theAs₂ O₃ coprecipitates with the crystals. Naturally, the raw materialneed not be arsenide, which is leachable under oxidizing conditions, butit can just as well as arsenate, which is extractable with sulfuric acidsolution alone, without oxidation.

The metal sulfates obtained can be used as raw materials for theproduction of pure metals or pure metal salts. The arsenic obtained isalso a valuable commodity, and especially if the arsenic has beenremoved by extraction, the H₃ AsO₄ solution obtained after theevaporation of the solution is an excellent initial material for theproduction of saturation agents and pesticides.

The invention is described below in more detail with the aid of anexample.

EXAMPLE

Precipitate obtained from the solution purification of the zinc processwas leached as follows:

    ______________________________________                                        Solid content     =           200 g/l                                         Temperature       =           95° C.                                   Initial H.sub.2 SO.sub.4 concentration                                                          =           300 g/l                                         Final H.sub.2 SO.sub.4 concentration                                                            =           120 g/l                                         ______________________________________                                    

Air was injected into the solution and dispersed well.

After six hours the color of the leach residue had turned white and theleach was discontinued. The results obtained:

    __________________________________________________________________________            Cu   Co   Ni   Zn   Cd   As   Pb   H.sub.2 O.sub.4                    __________________________________________________________________________    Precipitate                                                                   before leach                                                                          45.2 3.6  4.2  6.0  0.28 16.5 5.3  %                                  Precipitate                                                                   after leach                                                                           0.14 <0.025                                                                             <0.025                                                                             0.13      0.2  30   "                                  Leaching                                                                      yield   99.9 >99.9                                                                              >99.9                                                                              99.6      99.7 --   "                                  Solution                                                                      before leach                                                                          28   17   27   36   2    120       300 g/l                            Solution                                                                      after leach                                                                           118  24   35   48   2.6  153       120 g/l                            __________________________________________________________________________

The solution fed into the leach had been obtained by running it a fewtimes through the process according to the flow diagram shown in thefigure, and thus the example illustrates the results obtained in acontinuous-working process. Sulfuric acid was added to the solution at200 g H₂ SO₄ /l, the solution was then cooled to 30° C. and maintainedat this temperature for 6 hours. The crystals produced were removed byfiltering and then washed with a small quantity of water (0.2 ml H₂ O/lg of precipitate). The following results were obtained:

    ______________________________________                                                 Cu   Co     Ni     Zn  Cd  As   Pb  H.sub.2 O.sub.4                  ______________________________________                                        Solution before                                                               crystallization                                                                          118    24     35   48  2.6 153  --  120 g/l                        Solution after                                                                crystallization                                                                          26     17     26   37  2.1 152  --  320 g/l                        Precipitation Me                                                                         102    100    113  92  83           %                              Leached Me                                                                    Analysis of                                                                   crystals   18     1.4    1.7  2.4 0.1 0.1  2   %                              Coprecipitation                                                               of arsenic                            1.5                                     (calculated on the basis of arsenic content)                                  in the arsenide precipitate)                                                  ______________________________________                                    

Half of the solution was extracted using tributyl phosphate.

In order to extract arsenic from the solution obtained from thecrystallization of metal sulfates, an extraction agent was preparedwhich contained tributyl phosphate 85% by vol. and Shellsol K (analiphatic hydrocarbon with a distillation range of 194°-251° C. and aspecific weight of 0.79 g/cm³) 15% by vol.

One part (by vol.) of the solution obtained from the crystallization and1.4 parts (by vol.) of the tributyl phosphate solution used previouslyfor the arsenic extraction in question and then regenerated with waterwere stirred for 5 minutes at 30° C. The separated organic phaseobtained was stirred for an equal length of time at the same temperaturewith one tenth of its volume of water, whereafter the again separatedorganic phase was mixed in the same manner with half of its volume ofwater.

The analysis values of the aqueous solutions have been compiled in thetable below.

    __________________________________________________________________________              Cu  Co  Ni  Zn  Cd  As  Pb                                                                              H.sub.2 SO.sub.4                          __________________________________________________________________________    Solution after        g/l                                                     crystallization                                                                         26  17  26  37  2.1 152 --                                                                              320                                       Solution after                                                                extraction                                                                              27  17  25  38  2.1 76  --                                                                              228                                       Solution obtained                                                             by a phase volume                                                             ratio of 10                                                                             0.20                                                                              0.30                                                                              0.25                                                                              0.15                                                                              0.01                                                                              128 --                                                                              292                                       Solution obtained                                                             by a phase volume                                                             ratio of 2                                                                              0.014                                                                             0.013                                                                             0.011                                                                             0.010                                                                             0.001                                                                             86  --                                                                              66                                        __________________________________________________________________________

It can be seen from the table above that arsenic can be extracted highlyselectively. The trace amounts of coextracted metals can be removedsimply by washing the extraction agent with a small quantity of water.

On the other hand, considerable quantities of arsenic were coextracted.Calculations show that the separation factor

    S.sub.As,H.sbsb.2.sub.SO.sbsb.4 =C.sub.As /C.sub.As /C.sub.H.sbsb.2.sub.SO.sbsb.4 /C.sub.H.sbsb.2 SO.sbsb.4

where C_(As) and C_(H).sbsb.2_(SO).sbsb.4 are the concentrations ofarsenic and sulfuric acid in the organic phase, C_(As) andC_(H).sbsb.2_(SO).sbsb.4 the concentrations of arsenic and sulfuric acidin the aqueous phase, is in the order of 2.5 for both the extraction andthe water wash. For an expert it is thus evident that the extraction canbe performed in such a manner that a nearly sulfate-free arsenic acidproduct is obtained. By a countercurrent extraction of the above sulfatesolution in, for example, two extraction stages, and by a countercurrentwash in four washing stages, and by the combining of the obtained washsolution with the sulfate solution fed into the countercurrentextraction, an arsenic acid product is recovered which has a sulfatecontent of less than 10% of the arsenic content of the product. Theextraction and wash experiment was repeated by the process describedabove, the only difference being that iron had been added to the sulfatesolution in the form of ferric sulfate. The analysis showed that afterthe addition of iron the sulfate solution contained iron 25 g/l. Withinthe limits of the precision of the analysis, the extraction and washresults were the same as above. As regards iron, the results were:

    ______________________________________                                        Solution after extraction                                                                            24 g/l Fe                                              Wash solution obtained with                                                   a phase volume ratio of 10                                                                           2.2 g/l Fe                                             Wash solution obtained with                                                   a phase volume ratio of 2                                                                            0.16 g/l Fe                                            ______________________________________                                    

The results show that in spite of the fact that iron dissolves at asomewhat higher rate than other metals experimented with, iron can alsobe removed effectively from the extraction agent by means of a smallquantity of water. The possibility of obtaining an iron-free H₃ AsO₄product is significant in, for example, the production ofarsenic-bearing saturation products and pesticides.

One half of the solution was fed into a reactor, where sulfur dioxidewas injected through the solution, the temperature being maintained at60° C.

After ten hours the As⁵⁺ concentration had dropped to 26 g/l, and an As₂O₃ precipitate had been produced. The solution was cooled to 30° C. andthe solution was removed by decanting. The solution, which containedNaOH 60 g/l, was fed into the reactor and stirred until the precipitatehad dissolved completely. Before the leaching a sample was taken of As₂O₃, and this sample was washed with a small quantity of water.

Results:

    __________________________________________________________________________            Cu  Co Ni Zn Cd  As.sup.5+                                                                         As.sup.3+                                                                         H.sub.2 SO.sub.4                             __________________________________________________________________________    Solution before                                                               reduction                                                                             28  17 27 36 2   220 3   300 g/l                                      Solution after                                                                reduction                                                                             28  17 27 36 2   26  7   405 g/l                                      As.sub.2 O.sub.3 yield   88      5   %                                        As.sub.2 O.sub.3 analysis                                                             0.006                                                                             0.01                                                                             0.02                                                                             0.02                                                                             0.001   75.5                                                                              0.03                                         Coprecipitation                                                               of metal as                                                                   a percentage of                                                               the quantity                                                                  entering the                                                                  process 0.01                                                                              0.3                                                                              0.5                                                                              0.3                                                                              0.3     588                                              __________________________________________________________________________

The solution was returned to the leach of raw material.

As can be seen from the above, more arsenic was obtained from thesolution as As₂ O₃ then entered the leach. This means that only part ofthe solution needs to be fed through arsenic removal. Thus a higharsenic concentration can be stored in the solution by circulating itbetween the leach and the crystallization. This is advantageous for thenext arsenic removal and in any case it is possible to obtain anarsenic-free metal sulfate.

What is claimed is:
 1. A process for the treatment of raw materialswhich contain arsenic and metal to produce a metal-free arsenic productand an arsenic-free metal sulfate, comprising leaching the raw materialwith an aqueous solution of sulfuric acid at elevated temperature underoxidizing conditions; selectively separating an arsenate-bearing aqueoussolution; cooling the solution to crystallize the metals as sulfatesfrom the separated aqueous solution; separating the aqueous solutionfrom the metal sulfate crystals; removing arsenic from at least part ofthe aqueous solution, and finally recycling the solution to the leach.2. The process of claim 1, in which sulfuric acid is added to thesolution cycle in such a quantity that the sulfuric acid concentrationin the aqueous solution in the raw material leach is at least 100 g/l.3. The process of claim 2, in which sulfuric acid is added to thesolution is such a guantity that the sulfuric acid concentration in theaqueous solution in the raw material leach is 120-300 g/l.
 4. Theprocess of claim 1, in which the leach is performed at about 50°-100° C.5. The process of claim 1, including dispersing air or oxygen into amixture of an aqueous solution of sulfuric acid and metal arsenide toprovide said oxidizing conditions in said leaching step.
 6. The processof claim 1, in which the arsenic is removed from the arsenate-bearingaqueous solution by extracting the aqueous solution with tributylphosphate or dibutylbutyl phosphate dissolved in hydrocarbon and bystripping the organic phase with water.
 7. The process of claim 1, inwhich the arsenic is removed from the arsenate-bearing aqueous solutionby reducing the arsenate to arsenite by means of sulfur dioxide atelevated temperature in order to precipitate and separate the arsenic asAs₂ O₃.
 8. The process of claim 1, in which sulfuric acid is added tothe solution cycle at the metal sulfate crystallization stage.
 9. Theprocess of claim 1, in which the metal is Cu or Ni, and includingperforming the leach at a temperature of about 90°-100° C.
 10. Theprocess of claim 1 in which the metal is Co, and including performingthe leach at a temperature of about 65°-70° C.